Self-regulating electrically semi-conductive compositions, in the form of extruded flexible electrical heating cables, are often used in resistive heating applications. For example, heating cables incorporating these compositions may be used for freeze protection of pipes and for maintenance of flow characteristics of viscous fluids in pipes and storage containers. Self-regulating semi-conductive compositions may also be found in applications not involving resistive heating, for example, heat sensing and circuit-breaking.
In resistive heating applications, it is desirable for the self-regulating electrically semi-conductive composition to have a positive temperature coefficient of electrical resistance. A material exhibits a positive temperature coefficient of electrical resistance when the electrical resistance of the material increases as the temperature of the material increases. The increase in temperature may result from either a rise in the ambient temperature surrounding the composition or by reason of resistive heating caused by the passage of electrical current through the composition. One popular class of self-regulating compositions which exhibit positive temperature coefficients of resistance are thermoplastic compositions comprising electrically conductive particles, such as carbon black, dispersed throughout a polymeric base. The resulting composition may be viewed as a polymeric matrix foundation within which is located an interconnected array of conductive channels formed from the carbon particles.
It has been theorized that the positive temperature coefficient of electrical resistance of these compositions is caused by the expansion of the polymeric matrix at a rate which is greater than the rate of expansion of the conductive channels. The expansion of the polymeric matrix causes an increase, or other alteration, of the spacial relationship between the electrically conductive particles in a manner which causes the electrical resistance of the polymeric composition to increase. This increase in the electrical resistance (R) of the polymeric composition, for a fixed electrical potential (V) placed across the composition, causes the electrical current (I) passing through the composition to be reduced. Thus, the amount of heat generated by the passage of the electrical current through the resistive composition, given by the relationship that heat equals I.sup.2 R (or equivalently V.sup.2 /R), is also reduced. Conversely, a decrease in the temperature of the matrix causes the matrix to contract which places the conductive particles or channels in closer proximity to one another. This reduced spacing between conductive channels decreases the electrical resistance (R) of the polymeric composition which in turn causes the electrical current (I) to increase with a corresponding increase in heat generation.
An alternate theory, which does not depend on the expansion and contraction of the polymeric composition, explains the positive temperature coefficient of electrical resistance in terms of the amount of crystallinity present in the polymeric composition. According to this theory, the increase in the electrical resistance of the composition as the temperature of the composition increases may arise as a result of the reorientation of the crystalline-amorphic boundaries within the polymeric composition. This reorientation of the boundaries tends to electrically insulate the conductive particles (or groups of electrically conductive particles) from each other. The more effective insulation of the individual conductive components of the composition on the microscopic level contributes to the increase in the electrical resistance of the composition on the macroscopic level.
Additional information on the general theory of how self-limiting devices work may be found in U.S. Pat. No. 4,200,973 entitled "METHOD OF MAKING SELF-TEMPERATURE REGULATING ELECTRICAL HEATING CABLE" issued to Farkas; U.S. Pat. No. 3,914,363 entitled "METHOD OF FORMING SELF-LIMITING CONDUCTIVE EXTRUDATES" issued to Bedard et al.; and U.S. Pat. No. 3,823,217 entitled "RESISTIVITY VARIANCE REDUCTION" issued to Kampe.
Methods of making self-regulating positive temperature coefficient polymeric compositions generally comprise a variety of steps. The method steps often include: extruding the compositions; applying shape retaining jackets to the compositions; annealing the compositions at or above their melt point temperatures; and cross-linking the polymeric components with radiation. These steps, in a variety of combinations, are typical of procedures used in the production of self-regulating semi-conductive polymeric compositions containing amounts of carbon black ranging from less than about 10% to greater than about 75% of the total weight of the composition.
Electrically conductive polymeric compositions that contain greater than about 25%, by volume, of carbon black, have been described as having positive temperature coefficients of resistance and ar suggested for use as self-regulating heaters. An example of such compositions can be found in Kohler's U.S. Pat. No. 3,243,753 entitled "RESISTANCE ELEMENT" wherein the electrically semi-conductive compositions are described as containing 25 to 75 percent by volume carbon black as a result of in-situ polymerization. The method described therein results in a matrix of finely divided carbon particles embedded within a thermoplastic material. This is achieved by subjecting a mixture of the thermoplastic material and the carbon particles to elevated temperatures and pressures so that in-situ polymerization of the thermoplastic material occurs. Although such compounds may be useful for some heating purposes, it has been found that polymeric compositions containing more than about 25% by weight of carbon black generally possess poor cold temperature properties; exhibit inferior elongation characteristics; and generally do not possess good electrical current regulating characteristics in response to changes in temperature.
It has also been proposed that electrically semi-conductive compositions must not have more than 15% by weight of carbon black in order to provide a useful self-regulating heating device. Such teaching can be found, for example, in U.S. Pat. No. 3,793,716 entitled "METHOD OF MAKING SELF LIMITING HEAT ELEMENTS", issued to Smith-Johannsen. Described therein is a process for making a self-regulating heating element comprising polyethylene and less than 15% by weight of carbon black. This composition is manufactured by casting the semi-conducting composition from a solution or fusing a powder.
In U.S. Pat. No. 3,861,029 entitled "METHOD OF MAKING HEATER CABLE" issued to Smith-Johannsen, a polymeric material containing not more than about 15% by weight of carbon black is subjected to a prolonged annealing procedure at or above the melting temperature of the polymeric material. Articles produced in this manner exhibit electrical volume resistivities at room temperature in the range of from about 5 to 100,000 ohm-cm.
A further example of low carbon black content materials can be found in U S. Pat. No. 3,914,363 entitled "METHOD OF FORMING SELF-LIMITING CONDUCTIVE EXTRUDATES" issued to Bedard. This reference describes a method wherein a shape-retaining thermal plastic jacket is disposed about a self-regulating conductive article comprising crystalline polymeric compositions containing not more than about 15% by weight of conductive carbon black. The jacketed article is annealed at a temperature at or above the crystalline melting point of the composition, its shape being maintained by the jacket during the annealing process. The annealing procedure reduces the room temperature electrical volume resistivity of the polymeric composition to within the range of from about 5 to about 100,000 ohm-cm. Similarly, U.S. Pat. No. 3,858,144 entitled "VOLTAGE STRESS-RESISTANT CONDUCTIVE ARTICLES" issued to Bedard describes a method of making carbon black containing resistive heaters which are self regulating. The method disclosed comprises extruding a carbon black containing matrix preferably having less than 15% carbon black onto spaced apart electrodes; covering the extruded article with a shape-retaining jacket; annealing the article at a temperature at or above the melting point of the polymeric matrix; and radiation cross-linking the matrix to achieve thermal stability. Additionally, U.S. Pat. No. 4,277,673 entitled "ELECTRICALLY CONDUCTIVE SELF-REGULATING ARTICLE" and U.S. Pat. No. 4,327,480 entitled "ELECTRICALLY CONDUCTIVE COMPOSITION, PROCESS FOR MAKING AN ARTICLE USING SAME" both issued to Kelly, describe methods for making self-regulating compositions comprising extruding a polymeric composition; covering the extruded article with shape retaining jacket; annealing or thermal structuring of the material; and radiation cross-linking.
A method for increasing the stability of a device comprising at least one electrode and a conductive polymer is described in U.S. Pat. No. 4,426,339 entitled "METHOD OF MAKING ELECTRICAL DEVICES COMPRISING CONDUCTIVE POLYMER COMPOSITIONS" issued to Kamath. In this method, a composition containing about 15% to 17% carbon black is hot extruded onto a heated conductor to improve the contact between the conductor and the composition; the article is then annealed to decrease the resistivity of the composition; and either chemically cross-linked simultaneously with the extrusion and annealing or subsequently radiation cross-linked as a separate step after the extruding and annealing steps.
U.S. Pat. No. 3,823,217 entitled "RESISTIVITY VARIANCE REDUCTION" issued to Kampe describes a cyclic annealing process. Self-temperature regulating articles which contain carbon black dispersed therein in an amount not greater than about 15% by weight to the total weight of the composition are exposed to successive thermal cycles. During each cycle, the article is brought to a temperature which is at or above the melting temperature of the crystalline polymeric matrix in which the carbon black is dispersed. This process is used to reduce the electrical volume resistivity of the article to a value within the range of from about 5 to about 100,000 ohm-cm at 70.degree. F. for the low carbon black content compositions disclosed therein.
As described in Farkas, U.S. Pat. No. 4,200,973, a method of making a self-regulating heater using polymeric compounds containing from 17% to 25% carbon black comprises the following steps: a) extruding the composition around at least two substantially parallel spaced apart electrodes; b) placing a radiation penetrable shape retaining covering around the extruded composition and conductors; c) annealing the covered composition at a temperature that is at least at the melt point of the composition; d) cross-linking the annealed composition with radiation; and e) annealing the radiation cross-linked composition at a temperature which is at least at the melt point of the composition. Compositions produced by this method exhibit a positive temperature coefficient of electrical resistance. When combined with two or more spaced apart electrical conductors, these compositions provide a flexible, self-temperature regulating electrical heating cable having good current limiting properties and good physical properties. After the extrusion step a), the low carbon black content composition has an electrical resistance which is much too high for practical use as a heating device. The first anneal step c) reduces this resistance to a usable level. Since the first annealing step c) is at least at the melt point temperature of the composition, it is necessary to apply the shape retaining covering over the cable prior to the annealing process to prevent the heating cables from losing their shape during the annealing process. Therefore, the shape retaining covering must be capable of maintaining its shape at temperatures above the annealing temperature. After the first anneal step c), the composition still exhibits poor current-limiting features as well as poor physical properties. The current-limiting and physical properties of the composition are improved by the subsequent irradiation cross-linking step d) and the post irradiation annealing step e).
In this method and other methods which require annealing at or above the melt point temperature of the composition to reduce the resistance of the heater after it has been extruded, a non-melting shape-retaining jacket may be used to prevent the semi-conductive composition from deforming during annealing. This is necessary since the annealing is performed at a temperature at or above the melting point of the composition.
The application of the shape-retaining jacket and the selection of the material comprising the jacket are often difficult and limiting steps in these methods of producing self-regulating heater cables. Accordingly, it is desirable to eliminate the step requiring the application of the shape-retaining jacket while maintaining the other advantageous characteristics of a heater cable using a semi-conductive composition comprising carbon black in a range of from about 15% to about 25%. Additionally, it is desirable to reduce the number of annealing steps, since annealing is an expensive and time consuming process.